1. Field of the Invention
The present invention relates to semiconductor wafers having deformation ground in a defined way, and a method of producing such semiconductor wafers.
2. The Prior Art
In the production of semiconductor wafers, appreciable effort is expended on obtaining wafers which are as flat as possible. Only an extremely flat wafer surface ensures that the photolithographic application of electronic components to the wafer surface is achieved in a fault-free manner in the integration density which is now standard. Starting with the sawing of a wafer out of a rod-like single crystal and proceeding through to the surface finishing of the semiconductor wafer by lapping and polishing methods, the emphasis is therefore on obtaining semiconductor wafers having at least one plane surface.
In the further course of the processing of a semiconductor wafer, however, method steps are generally necessary which impair the flatness of the semiconductor wafer achieved up to that point. Because of the circular wafer shape, a rotationally symmetrical deformation of the semiconductor wafer is frequently to be observed after a single-sided treatment of a wafer surface. Thus, for example, the application or creation of one or more material layers (for example, by epitaxy or oxidation) on a preferred wafer surface results in stresses in the semiconductor material which cause a rotationally symmetrical deformation of the semiconductor wafer. A similar deformation occurs if a semiconductor wafer is etched, doped or subjected to a so-called damage treatment (the creation of defects in the crystal lattice) on one side. A precise inspection shows a semiconductor wafer treated in this way to be bowed.
A recognized measure of the deformation of a semiconductor wafer is the "warp." The warp specifies the difference between the maximum and the minimum distance of the center plane of the semiconductor wafer from a reference plane. It can be determined, for example, in a method of measurement in accordance with the U.S. Standard ASTM F 657-80. The one-sided treatment of a semiconductor wafer described above increased the warp. For ideally flat and plane-parallel wafer surfaces, the warp is equal to zero, unless the semiconductor wafer was already deformed in a defined way prior to the one-sided treatment so as to virtually or completely compensate for the additional rotationally symmetrical deformation. In the latter case, the warp would become smaller and the flat wafer geometry necessary for a fault-free photolithographic application of electronic components would be achieved.
The total thickness variation (TTV value) of a semiconductor wafer is particularly suitable for assessing the parallelism of the wafer surfaces. Referred to as the TTV value is the absolute amount of the thickness values of a semiconductor wafer determined from a multiplicity of point measurements. The TTV value does not therefore change if a semiconductor wafer deforms as a consequence of a one-sided treatment of its surface. A prerequisite for this is, however, that the treatment does not alter the wafer thickness non-uniformly.
German Published Application No. 3,906,091 A1 and corresponding U.S. Pat. No. 4,991,475, disclose that deformed semiconductor wafers can be created by influencing certain parameters during the cutting of semiconductor wafers from crystal rods by means of annular saws. However, the precision of this method is not sufficient to consistently reproduce semiconductor wafers having the same identical defined deformation or warp.